The present invention relates to a method of operating a burner, which has at least one first fuel supply conduit with a first group of fuel outlet openings, essentially arranged in the direction of a burner longitudinal axis, for the introduction of a first premix fuel quantity into a swirl space and one or a plurality of second fuel supply conduits with a second group of fuel outlet openings essentially arranged in the direction of the burner longitudinal axis, it being possible to admit fuel to the second fuel supply conduits independently of the first fuel supply conduit. The invention also relates to a burner which can be advantageously operated by means of the method. The combustion spaces of gas turbines are a preferred field of employment for such burners; in addition such burners are, for example, also employed in atmospheric boiler firing systems.
A conical burner consisting of a plurality of shells, a so-called double-cone burner, is known from EP 0 321 809. A swirl flow in the interior space of the cone enclosed by the conical partial shells is generated by the conical swirl generator composed of a plurality of shells. Because of a cross-sectional step at a combustion-space end of the burner, the swirl flow becomes unstable and merges into an annular swirl flow with reverse flow at the core. This reverse flow permits stabilization of a flame front at the burner outlet. The shells of the swirl generator are combined in such a way that tangential air inlet slots for combustion air are formed along the burner longitudinal axis. Supply conduits for a gaseous premix fuel are provided at the inlet flow edge of the conical shells formed by this means. These supply conduits have outlet openings, distributed in the direction of the burner longitudinal axis, for the premix gas. The gas is injected transverse to the air inlet gap through the outlet openings or holes. In association with the swirl, generated in the swirl space, of the flow of combustion air and fuel gas flow, this injection leads to good mixing of the fuel gas or premix gas with the combustion air. In such premix burners, good mixing is the precondition for low NOx values during the combustion process.
For further improvement to such a burner, a burner for a heat generator is known from EP 0 780 629, which burner has an additional mixing section, which abuts the swirl generator, for further mixing of fuel and combustion air. This mixing section can, for example, be embodied as a downstream tube, into which the flow emerging from the swirl generator is transferred without appreciable flow losses. By means of this additional mixing section, the degree of mixing can be further increased and, therefore, the pollutant emissions reduced.
WO 93/17279 shows a further known premix burner, in which a cylindrical swirl generator with an additional conical inner body is employed. In this burner, the premix gas is likewise injected into the swirl space by means of supply conduits with corresponding outlet openings, which are arranged along the axially extending air inlet slots. In the conical inner body, this burner has, in addition, a central supply conduit for fuel gas, which can be injected for pilot operation into the swirl space near the outlet opening of the burner. This additional pilot stage is used for starting the burner. The supply of the pilot gas in the outlet region of the burner leads, however, to increased NOx emissions because it is only inadequate mixing with the combustion air which can take place in this region.
EP 0918191 A1 shows a burner, of the generic type, for operating a heat generator which, parallel to a first supply conduit for fuel, also has a second supply conduit for another type of fuel, which supply conduit is matched to the other type of fuel. The two supply conduits can be initiated independently of one another. By means of this design, the burner can be operated, without modification, on different types of fuel.
In all the burners presented, the injection of the premix gas in the air inlet gap takes place by means of supply conduits with outlet openings essentially arranged in the direction of the burner longitudinal axis. In consequence, the characteristics of the injection are predetermined with respect to penetration depth, mixing of the gas jets and the fuel distribution along the air inlet slots or the burner longitudinal axis. The arrangement of the outlet openings has therefore already determined the quality of mixing of the gas and the combustion air and the fuel distribution at the burner outlet. These parameters are, in turn, decisive for the NOx emissions, for the flame-out and flash-back limits and for the stability of the burner with respect to combustion pulsations.
In the case of different loads, gas qualities or gas preheat temperatures, however, different upstream gas pressures occur at the outlet openings and these, in turn, lead to different premixing conditions and mixture qualities at the fuel outlet. The different premixing conditions then result in different emission values and stability conditions, which depend on the load, the gas quality and the gas preheating. The known burners can therefore only be operated optimally for quite specific value ranges of these parameters.
A problematic feature in the operation of premix burners, particularly in gas turbines, is the part-load range because, in this range, the combustion air is mixed with only comparatively small fuel quantities. In the case of the complete mixing of the fuel with the whole of the air, however, a mixture occurs which is no longer capable of being ignited, particularly in the lower part-load range, or is only capable of forming a very unstable flame. This can lead to damaging combustion pulsations or to the flame becoming completely extinguished.
In order to match the known burners to certain emission values or to a certain stability window in the case of different loads, environmental conditions, gas qualities and preheat temperatures, the possibility currently exists of, on the one hand, staging the premix gas supply to individual burner groups in cases where multiple burner arrangements are employed. This, however, is only possible in the case of multi-row burner arrangements. In the case of single-row annular combustion chambers, this technology has the disadvantage that a temperature profile, which is non-uniform in the peripheral direction, appears at the combustion chamber outlet.
Another possibility, as already sketched above, is to equip burners with a so-called pilot fuel supply. The burners are then operated as diffusion burners at very high excess air numbers. This results, on the one hand, in superior flame stability but, on the other, in high emission values and further technical disadvantages in operation.
The object of the present invention consists in providing a burner operating method and a burner, by means of which the burner can, as far as possible, be stably operated in premix operation at approximately constant NOx emission values, even in the case of changes to the load, the gas quality or the gas preheat temperature.
The object is achieved by means of the method according to claims 1 and 7 and the burner according to claim 8. Advantageous designs and developments of the burner and the method are the subject matter of the subclaims.
In the present method, a burner with swirl body and swirl space is employed which has at least one first fuel supply conduit, with a first group of fuel outlet openings essentially arranged in the direction of a burner longitudinal axis, for the introduction of a first premix fuel quantity into the swirl space and one or a plurality of second fuel supply conduits with a second group of fuel outlet openings essentially arranged in the direction of the burner longitudinal axis, it being possible to admit fuel to the second fuel supply conduits independently of the first fuel supply conduit. In order to operate the burner, the supply of the fuel via the first fuel supply conduits is controlled, in an open-chain or closed-loop manner, separately from the supply of the fuel via the second fuel supply conduits, the same fuel being supplied to the first and second fuel supply conduits. By controlling the mass flow ratio between the first fuel quantity supplied via the first fuel supply conduits and a fuel quantity supplied via the second fuel supply conduits during the operation of the burner, the burner can be stably operated with approximately constant NOx emission values even in the case of changes to the load, the gas quality or the gas preheat temperature.
In the preferred embodiment, the fuel is then employed as a premix fuel and is divided at variable mass flow ratio between the first and second supply conduits. The feed of premix fuel differs from the feed of pilot fuel, i.e. of fuel for realizing a pilot stage, in that premix fuel is introduced into the swirl space with a higher inertia, preferably transverse to the flow of the combustion air. When, on the other hand, the fuel is introduced as pilot fuel, the burner is operated in a diffusion mode.
The fuel is preferably introduced into the burner in such a way that it is distributed between the first and second fuel supply conduits as a function of the load.
In a further preferred mode of operation of the burner, in a first operating condition, the whole of the fuel quantity is essentially supplied via the first fuel supply conduit or conduits and is introduced into the combustion airflow via the first group of fuel outlet openings and, in a further operating condition, at least a part of the total fuel quantity is introduced into the combustion airflow via at least one of the second fuel supply conduits with the second group of fuel outlet openings.
If the burner is operated in a heat generator, the total fuel can, in a partial load condition of the heat generator, be supplied via the first fuel conduits and, in full-load operation of the heat generator, the fuel can be divided between the first fuel supply conduits and one or a plurality of second fuel supply conduits.
In addition to the above-mentioned load-dependent distribution of the fuel between the first and second fuel supply conduits, the distribution can also be controlled according to other operating parameters. As an example, the fuel can also be distributed between the first and second fuel supply conduits as a function of measured combustion chamber pulsations of a gas turbine, of pollutant emissions, of measured material temperatures, of the flame position recorded by a flame position sensor or of other measured or operating parameters.
The one or a plurality of second fuel supply conduits, by means of which the quantityxe2x80x94and therefore also the upstream fuel pressurexe2x80x94of premix fuel which is injected into the swirl space via the second group of fuel outlet openings can be set independently of the quantity of premix fuel which flows via the first fuel supply conduits, make possible a simple matching of the mixture distribution and the mixture quality to different boundary conditions. In addition, this design also makes it possible to achieve compensation for different Wobbe indices by, for example, the first fuel supply conduits supporting a certain power or a certain volume flow and the rest of the power or the volume flow being operated by means of the second fuel supply conduits. The axial and radial fuel distribution in the burner can be favorably influenced by appropriate arrangement of the second fuel supply conduits, with the corresponding second group of fuel outlet openings, relative to the first fuel supply conduits, with the first group of fuel outlet openings. It is therefore possible to achieve a specified enrichment of the mixture with fuel in certain regions of the burner outlet, during part-load operation, in order to improve the flame stability. At high burner load, the fuel can then be uniformly distributed, which results in low emissions.
By means of a design, in which premix fuel can also be admittedxe2x80x94and is admittedxe2x80x94to a plurality of second fuel supply conduits independently of one another, an even more finely staged matching of the mixture distribution and the mixture quality to different boundary conditions can be undertaken.
In addition, the invention also includes designs such as those in whichxe2x80x94in addition to first and second fuel supply conduitsxe2x80x94third, fourth etc fuel supply conduits are also present and can have fuel admitted to them independently.
The present burner consists of a swirl generator for a combustion airflow, a swirl space and means for introducing fuel into the combustion airflow, the swirl generator having combustion air inlet openings for the combustion airflow entering tangentially into the swirl space, which comprise means for introducing fuel into the combustion airflow of one or a plurality of first fuel supply conduits with a first group of fuel outlet openings, essentially arranged in the direction of a burner longitudinal axis, for a first premix fuel quantity and the burner has one or a plurality of second fuel supply conduits with a second group of fuel outlet openings essentially arranged in the direction of the burner longitudinal axis, for a second fuel quantity, preferably a premix fuel quantity, it being possible to admit fuel to these second fuel supply conduits independently of the first fuel supply conduit or conduits. In the preferred variant described, the burner is characterized by an inner body being arranged in the swirl space, the fuel outlet openings of at least one second fuel supply conduit being arranged on the inner body, essentially distributed in the direction of the burner longitudinal axis. In a preferred embodiment, the inner body is a fuel lance, which is arranged in the swirl space on the burner longitudinal axis.
One or a plurality of the first groups of fuel outlet openings are preferably arranged in the region of at least one of the combustion air inlet openings.
In the present application, an arrangement essentially in the direction of the burner longitudinal axis is to be understood as an arrangement on longitudinal axes which extend parallel to or at an angle of  less than 45xc2x0 to the burner longitudinal axis.
In a possible embodiment of the present burner, some of the second fuel supply conduits are also arranged immediately adjacent to the first fuel supply conduits, preferably parallel to the latter. In this arrangement, at least one second fuel supply conduit should be provided adjacent to each first fuel supply conduit.
It is, however, obvious per se that the second fuel supply conduits can also be provided in symmetrical arrangement on the swirl generator, independently of the first fuel supply conduits. In this case, the geometry of the swirl generator is unimportant. As an example, conical swirl generators, such as are known from the publications, mentioned at the beginning, of the prior art, for example with two, four or more air inlet slots, can be employed. Other geometries, such as cylindrical swirl generators or cylindrical swirl generators with conical or cylindrical inner bodies can also be employed.
In one embodiment of the burner, some of the second fuel supply conduits are arranged on the outer shell of the swirl body and in particular, in this arrangement, on the air inlet slots along the latter. In the present burner, the essential feature is that the second fuel supply conduits have a plurality of fuel outlet openings, which are essentially distributed in the direction of the burner longitudinal axis, in order to permit the achievement of adequate premixing. The outlet openings arc usually located on longitudinal axes extending parallel to the burner longitudinal axis or on longitudinal axis at an angle to the burner longitudinal axis predetermined by a conical shape of the swirl generator or inner body.
Depending on the possibilities desired for influencing the premixing, the second fuel outlet openings of the second fuel supply conduits can have different distances between them or flow cross sections, as compared with the first fuel outlet openings. Particularly in the case of an arrangement in which at least one second fuel supply conduit is also provided immediately adjacent to a first fuel supply conduit, the respective fuel outlet openings can also have the same distances between them, but be arranged offset relative to one another. This leads to a uniform injection of the premix fuel into the swirl space. In addition, the first fuel outlet openings can, for example, be arranged over the whole of the axial extent of the combustion air inlet openings, but the second fuel outlet openings being only arranged within a certain partial axial region. In a similar manner, it is also possible to provide the first fuel outlet openings in a first axial partial region only and the second fuel outlet openings only in a second axial partial region abutting the first partial regionxe2x80x94or vice versa. Different possibilities for influencing the operation of the burner on the basis of these different design possibilities, to whose combination no practical limits are set, can be taken from the exemplary embodiments.
For mutually independent admission of the premix fuel to the first and the second fuel supply conduits, the latter are equipped with different connections. Additional means are preferably provided for the mutually independent closed-loop or open-chain control of the premix fuel supply to the first and the second fuel supply conduits. The different supply can, for example, be controlled by an suitable control valve.